The emission of particulate matter in exhaust from compression-ignition engines is regulated for environmental reasons. Thus, vehicles equipped with compression-ignition engines often include after-treatment components such as particulate filters, catalyzed soot filters and adsorption catalysts for removing particulate matter and other regulated constituents (e.g., nitrogen oxides or NOx) from their exhaust streams. Particulate filters and other after-treatment components can be effective, but can also increase back pressure as they collect particulate matter.
Particulate matter may include ash and unburned carbon particles generally referred to as soot. As this carbon-based particulate matter accumulates in the after-treatment components, it can increase back pressure in the exhaust system. Engines that have large rates of particulate mass emission can develop excessive back pressure levels in a relatively short period of time, decreasing engine efficiency and power producing capacity. Therefore, it is desired to have particulate filtration systems that minimize back-pressure while effectively capturing particulate matter in the exhaust.
To accomplish both of these competing goals, after-treatment components must be regularly monitored and maintained either by replacing components or by removing the accumulated soot. Cleaning the accumulated soot from an after-treatment component can be achieved via oxidation to CO2 (i.e., burning-off) and is known in the art as regeneration. To avoid service interruptions, regeneration is generally preferred over replacement of after-treatment components.
One way that regeneration may be accomplished is by increasing the temperatures of the filter material and/or the collected particulate matter to levels above the combustion temperature of the particulate matter. Elevating the temperature facilitates consumption of the soot by allowing the excess oxygen in the exhaust gas to oxidize the particulate matter. Particulate matter may also be oxidized, and thus removed, at lower temperatures by exposing the particulate matter to sufficient concentrations of nitrogen dioxide (NO2). Exhaust from a compression-engine, such as a diesel engine, typically contains NOx, which consists primarily of nitric oxide (NO) and approximately 5 to 20 percent NO2, with greater levels of NO2 being common where oxidation catalysts are present in the exhaust stream. Thus, some level of regeneration occurs even at relatively low temperatures.
The regeneration process can be either passive or active. In passive systems, regeneration occurs whenever heat (e.g., carried by the exhaust gasses) and soot (e.g., trapped in the after-treatment components) are sufficient to facilitate oxidation, and/or whenever sufficient concentrations of NO2 are present in the exhaust to enable oxidation at lower temperatures. In active systems, regeneration is induced at desired times by introducing heat from an outside source (e.g., an electrical heater, a fuel burner, a microwave heater, and/or from the engine itself, such as with a late in-cylinder injection or injection of fuel directly into the exhaust stream). Active regeneration can be initiated during various vehicle operations and exhaust conditions. Among these favorable operating conditions are stationary vehicle operations such as when the vehicle is at rest, for example, during a refueling stop. Engine control systems can be used to predict when it may be advantageous to actively facilitate a regeneration event and to effectuate control over the regeneration process.
An engine control system may use a soot model to deduce (i.e., predict) a mass of soot accumulated in the after-treatment component by monitoring properties of the exhaust stream as it flows through the after-treatment component. The control system can use the deduced soot mass data to monitor soot loading over time, to determine or anticipate when regeneration may be necessary or desirable, to facilitate a regeneration event, and/or to effectuate control over a regeneration process or other remedial measures. In one exemplary soot model, the pressure decrease across a loaded after-treatment component may be used, along with knowledge of the relationship between soot accumulation and pressure decrease, to estimate the extent of soot loading in the after-treatment component. This is possible because, as soot accumulates in an after-treatment component, the pressure decrease typically increases (at specific temperature and volumetric flow rates) from pressure decreases experienced when the after-treatment component is clean.
Because changes in temperature, pressure, and flow rate affect the pressure decrease experienced by exhaust as it passes through an after-treatment component, the accuracy and reliability of measurements for these parameters is important. Ideal gas laws may also be used to adjust flow rates for changing temperatures and pressures, further adding to the importance of accurate determinations for these parameters. Unfortunately, however, a number of difficulties have been encountered determining temperatures in and around after-treatment components. For example, experience has shown that exhaust gas temperatures can deviate significantly from material temperatures in an after-treatment component, particularly during non-steady, or transient, operation. This is due to significant thermal inertias that may exist in typical after-treatment components, which can be accompanied by correspondingly large temperature gradients as the components respond to transient operating conditions. Therefore, as a result of the large dependency on an accurate temperature measurement, errors can be caused by temperature gradients occurring in after-treatment components.
Accordingly, it is desirable to provide an improved system and method for determining when to facilitate active regeneration and for controlling active regeneration of particulate filtration systems, particularly having improved model accuracy in the presence of large temperature gradients occurring in and around after-treatment components.